The recent instances of death and injury of soldiers by "friendly fire" during Operation Desert Storm has all too poignantly stressed the need for the armed services to be able to accurately and rapidly identify those forces friendly to them. An accurate system of friendly force identification could have easily avoided much of the bloodshed from such unfortunate accidents.
Recent advances in surveillance technology have created a necessity for better systems for distinguishing friend from foe. In military applications, military aircraft and shipboard and ground stations are equipped with sophisticated radar systems which can detect the presence of other aircraft in its general vicinity long before anyone at the radar site can make a visual identification. The capability also exists at these radar sites, or at the command of the radar sites, for firing weapons at a radar target. In a combat context, there is a great need to make the earliest possible friend or foe determination of the detected aircraft. If the aircraft is fired upon before identification is made, there is a chance that friendly aircraft will be shot down. If too much time passes before a friend or foe determination is made, there is a chance of being attacked by enemy aircraft.
As previously stated and as illustrated by Desert Storm statistics, there is also a need for IFF capability for individual soldiers on combat missions. When numerous soldiers are deployed in territory in which there is a good chance that enemy soldiers will be present also, the opportunity for fratricide is great. A soldier who fires at any movement in a cotabat environment risks shooting friendly personnel. However, waiting until a positive visual identification is made puts the soldier at risk himself. This is particularly critical at night and in inclement weather, when visibility is accurate over a shorter distance. An IFF system for individuals is needed to avoid injury and death due to friendly fire. This system must be equally effective both during the day and at night, and also during adverse weather conditions. Because the system will be carried by an individual, it must be compact, lightweight, and easy to activate and deactivate. Such an IFF system would also be useful in signalling applications, such as in locating personnel downed in enemy territory.
Numerous IFF-type systems are known which are adapted for a variety of uses. Many of these systems involve an interrogator and a transponder. The interrogator "challenges" a person or vehicle with a coded signal. A friendly target having a transponder will recognize the coded signal as a challenge, or interrogation, and will transmit a properly coded reply to the interrogator. The interrogator will recognize the reply as friendly, and the user will act accordingly. An unfriendly target, or foe, will not recognize the coded interrogation, and will therefore not respond. The interrogator, upon receiving no reply after waiting a predetermined amount of time, will indicate to the user that the target is unknown and presumed hostile, and the user will act accordingly.
Such a system is disclosed in U.S. Pat. No. 4,814,769, issued to Robin et al. This is a classic IFF system utilizing an interrogator and a transponder. Other such systems are disclosed in U.S. Pat. No. 3,104,478 to Strauss et al., U.S. Pat. No. 4,814,742 to Morita et al., U.S. Pat. No. 4,837,568 to Snaper, U.S. Pat. No. 4,099,050 to Sauermann, U.S. Pat. No. 4,249,265 to Coester, U.S. Pat. No. 4,134,008 to de Corlieu et al., U.S. Pat. No. 4,325,146 to Lennington, U.S. Pat. No. 4,361,911 to Buser et al., U.S. Pat. No. 4,995,101 to Titterton et al., U.S. Pat. No. 5,001,488 to Joguet, and U.S. Pat. No. 5,142,288 to Cleveland. These systems all include an interactive transponder. That is, the transponder only transmits a reply when it senses an interrogation. At all other times, no output signal issues from the transponder.
This type of interactive system is advantageous in keeping the communication between friendly entities secure. During the time that the transponder does not output a signal, unfriendly observers cannot detect the transponder's presence. However, transponders that are not interactive, that is, those that continuously transmit, have much more simple designs. A continuously transmitting transponder, or beacon, only requires a transmitter. An interactive transponder requires a receiver f or the incoming interrogation, signal decoding and recognition circuitry and signal generation circuitry for the reply in addition to the transmitter. A beacon-type transponder requires a less expensive design, is less expensive to build, is more reliable due to the less complicated design, and can be designed in a smaller package. The interrogator making up part of a system using a beacon transponder would employ a simpler design as well. The interrogator need only be a detector in this case, as only a receiver and signal recognition circuitry would be required. If designed properly, an IFF system comprising a beacon and a detector can also provide the security manifest in the interactive type transponder.
U.S. Pat. No. 3,227,882, issued to Bassett et al. discloses a beacon and detector type signalling system. The output of the beacon is coded, but the coding is not used to verify the reply; the detector interprets the coded reply word to elicit information. U.S. Pat. No. 4,081,669 to Klingman, III discloses a recognition is system for robots. Beacons are placed on numerous objects and a robot is equipped with an optical detector. The beacon outputs are coded so that the robot can identify the objects. U.S. Statutory Invention Registration No. H333 to Curtis discloses an infrared beacon on a target vehicle, along with a special purpose receiver on a surveillance vehicle. Different filters may be used on the beacon to change the output frequency within the infrared range. The pulse repetition rate of the output signal may also be changed. The surveillance vehicle then recognizes as friendly any vehicle emitting a signal having the predetermined wavelength and pulse rate.
The preceding inventions do not take full advantage of the size and money saving advantages of the beacon-detector type IFF system, nor do they provide the security necessary for modern military applications. The most sophisticated coding method in the previous references, that used in the Curtis invention, is too easily corruptible using today's military technology. The beacon signal transmitted at any one time has a constant wavelength and a constant pulse repetition rate. Anyone with a detector sensitive to the particular frequency used will immediately recognize the pulse rate. The beacon output signal would then be too easy to detect and mimic by unfriendly observers. Also, the unit's size would make it suitable only for use with vehicles, being too cumbersome to be carried by individual personnel who need covert identification means on the battlefield.
Recently, a series of field tests were conducted by the U.S. Army Infantry School on various soldier identification systems in order to determine their utility as a means to prevent fratricide when employed by dismounted soldiers. One of these devices consisted of a near infrared (0.88 micrometer wavelength) light emitting diode ("LED") connected to a 9 volt battery and flashed at about 2 Hz. This wavelength is invisible to the naked eye, but falls within the responsivity range of current image intensifier ("I.sup.2 ") type detector devices.
The tests demonstrated that the system, when worn on a soldier's web gear and viewed from the front or side, was visible at ranges up to 1500 meters through currently fielded I.sup.2 equipment. When viewed from the rear, with the light obscured by the soldier's body, the device was detectable intermittently to 1500 meters, and was clearly visible to all, both friend and foe, up to a range of 700 meters (500 meters in the woods). Furthermore, if a soldier were standing near shrubbery, in a foxhole, or behind some object, enough infrared radiation was scattered from leaves, other nearby surfaces, or particles in the air that a "halo" was created that generally increased the device's signature. This provided a limited non-line of sight capability, and accounts for the visibility of the system's beacon from any angle, even when obscured by the soldier's body.
The tests therefore demonstrated that the system functioned as an ideal individual soldier identification device, but was too easily exploitable by potential adversaries. The frequency chosen and lack of adequate coding made it too susceptible to detection by imaging systems such as night vision equipment. For this reason, the system is not suitable for use in assisting the recognition of friendly dismounted troops or vehicles in a situation where security could be compromised.